1. Field of the Invention
The present invention relates to a rotary impact tool such as an impact wrench or an impact driver used for fastening or loosening of fastening member such as a screw, a bolt or a nut.
2. Description of the Related Art
In a rotary impact tool used for fastening a member to be fastened such as a bolt or a nut, it is desirable that a driving source is stopped for completing a fastening operation when a fastening torque reaches a predetermined set value. Although measurement of actual fastening torque is most desirable at a point of precision of fastening, it is necessary to provide a torque sensor in an output shaft of the rotary impact tool. It causes not only increase of cost and upsizing of the rotary impact tool but also decrease of usability. Thus, the fastening torque is estimated with various methods and the fastening torque is limited with the estimated value in the conventional rotary impact tool. In the conventional rotary impact tool, the motor serving as a driving source is normally rotated at the highest rotation speed, and the setting of the fastening torque depends on such an assumption. Therefore, the conventional rotary impact tool is suitable for fastening an object with heavy load. However, when the conventional rotary impact tool is used for fastening an object with a light load, the object, for example, a fastening member such as a bolt will be damaged by several impacts or over fastening may occur, even though the fastening torque is set to be the smallest value. Thus, the conventional rotary impact tool is rarely used for a work such as an interior finish work in which the finish precision is emphasized.
For limiting the fastening torque by estimation of the fastening torque, it is possible to simplify the stop control of the driving source by stopping the driving source when a counted number of impacts reaches a value previously set or a value calculated with a torque gradient after stopping normal rotation of a bit of the rotary impact tool. The actual fastening torque, however, is largely different from the desired fastening torque, so that damage to the object to be fastened due to over fastening or loose fastening members due to under fastening occurs.
In addition, it is proposed that a rotation angle of a fastening member as the fastening member is measured and the driving source is stopped when a rotation angle of the fastening member in each impact becomes equal to or smaller than a predetermined angle. Since the rotation angle of the fastening member is in inverse proportion to the fastening torque, such a rotary impact tool is controlled with fastening torque in theory. The rotary impact tool with a driving source moved by a battery, however, has a problem that the fastening torque largely varies due to voltage drop of the battery. In addition, it is largely affected by properties such as hardness or softness of the object to be fastened by the fastening member.
In another conventional rotary impact tool shown in Japanese Laid-Open Patent Publication No. 2000-354976, impact energy and rotation angle of a fastening member in each impact are sensed, and a fastening torque is calculated with using the impact energy and the rotation angle of the fastening member. When the calculated fastening torque becomes equal to or larger than a predetermined set value, the driving source is stopped. It is further shown that the impact energy is calculated with using a rotation speed of an output shaft at instant of impact of the output shaft and rotation speed of the output shaft just after the impact. Since the impact energy is calculated with the rotation speed of the output shaft at instant of impact, it needs a high resolution sensor and high speed processor which cause to increase of cost.